The present invention concerns a three-core cable, preferably intended for medium-level voltage, provided with a longitudinal water seal, which cable also comprises, in association with the three conductors that are included inside the sheath and screen, filler strings that are placed in cavities in the longitudinal direction of the cable at the outside of the conductors in order to impart to the cable its intended shape, whereby a longitudinal water seal is achieved with the aid of expansion tape.
One reason that a three-core cable is provided with a longitudinal water seal is a desire to prevent water from travelling long distances within the three-core cable following damage having occurred with respect to the outer sheath of the cable. The aim of this is to prevent as far as is possible the three insulated conductors of the cable being exposed to water or moisture. It is namely previously known that moisture in the PEX insulation of a cable can cause what is known as xe2x80x9cwater treeingxe2x80x9d, that is, weakening of the insulating capacity of the material used. It has been necessary to exchange many older cable throughout the years due to this phenomenon. It should however be noted that, while it is true that the PEX cables used earlier were constructed in a different manner to today""s cables, it is possible to produce under extreme conditions in the laboratory the said water treeing in cables that are currently used.
One of the differences between the old design and the current design concerns the outer semi-conducting layer that surrounds each conductor. Nowadays, this layer is extruded with semi-conducting rubber or semi-conducting PEX. The layer previously was varnished using a semi-conducting water emulsion, after which semi-conducting textile tape was applied over the layer. Semi-conducting tape was occasionally used on its own. Experience from the field with the current design is very good, but there is a certain amount of concern among customers that the problem of treeing will again arise.
Several suggestions have been made for the design of a three-core cable with a longitudinal water seal according to what is described in the introduction.
Accordingly, some of these prior art solutions are shown in FIGS. 1-3, which are primarily intended to illuminate the fact that a considerable amount of sealant material in the form of expansion tape is currently used in association with known three-core cables provided with a longitudinal water seal.
The three-core cables 10 according to FIGS. 1 and 2 are sealed in the longitudinal direction in essentially the same way and also have essentially the same design. The cables 10 thus comprise three identical conductors 12, each of which consists of an inner core 14, which is surrounded by the normal insulation 16. In order for the three-core cable 10 to obtain an essentially round shape, it is supplemented with three identical filler strings 18, 18xe2x80x2. According to FIG. 1, these filler strings are essentially round, while in FIG. 2 they are shaped in order to further improve the desired round shape of the three-core cable 10 and to reduce cavities. Outside of the package that is thus formed of conductors 12 and filler strings 18, 18xe2x80x2, a cable screen 20 of metal threads 22 is located, and the outer cable sheath 24 is located outside of the cable screen 20.
In the event of damage occurring with respect to the cable sheath 24, water or moisture can penetrate the cable 10, which, if the cable 10 were not provided with a longitudinal water seal, would entail the water spreading along long stretches of the cable 10. Cavities 26 that exist in the three-core cable 10 must therefore be sealed in the longitudinal direction in order to limit the spread of the water to as short a stretch as possible, in the best case to a few meters only.
In order to achieve the longitudinal water seal according to the known designs in FIGS. 1 and 2, each conductor 12 is now fully covered with surrounding expansion tape 28. The cable body itself is also externally provided with fully covering expansion tape 28, and, in addition, a further layer of expansion tape 28 may, in order to further increase the security, be placed outside of the cable screen 20.
Furthermore, expansion thread 30 is placed along the longitudinal direction of the cable in association with the cavities 26 that exist, in order to further improve the known designs.
The expansion tape 28 normally consists of a sealant in the form of polyacrylate, which is applied to a woven tape (not shown in the drawing). All tapes except for the one that is applied outside of the cable screen 20 must be of semi-conducting material in order to ensure electrical contact between the screen and the outer semiconductor. This material requirement entails, naturally, additional cost. It would be significantly cheaper if it were possible to use expansion tape 28 of non semi-conducting material.
In the arrangement according to FIG. 3, the expansion tape 28 is not wound around the conductors 12, as it is in the designs according to FIGS. 1 and 2, but is placed around these in the longitudinal direction. One advantage of this is that the operation concerning the winding of the expansion tape onto each conductor 12 is eliminated, but even in this case, semi-conducting expansion tape is required.
As is clear from the above description of the prior art solutions, all of these suggestions have the common property that they require very large consumption of material both with respect to the expansion tape that is used for the longitudinal water seal and with respect to the filler strings that are placed in direct connection with the three conductors. All of the currently known embodiments concerning three-core cables with a longitudinal water seal are thus, due to the large material consumption, very expensive to manufacture, and thus there is an accumulated need for a cable that can be manufactured more cheaply. This means that it requires less material consumption and is easier to manufacture than current cables of an equivalent type.
The aim of the present invention is primarily to make possible a considerable reduction of the amount of expansion tape that is used during manufacture of three-core cables.
Another aim is to make possible the use of a cheaper expansion tape material.
According to the invention, this is made possible with a three-core cable, preferably intended for medium-level voltage, provided with a longitudinal water seal, said cable comprising, in association with the three conductors (12) that are included inside the sheath and screen, filler strings (18xe2x80x3) that are placed in cavities in the longitudinal direction of the cable at the outside of the conductors in order to impart to the cable its intended shape, whereby the longitudinal water seal is achieved with the aid of expansion tape (28xe2x80x2), characterized in that the expansion tape is wound on all conductors and on all filler strings without covering them fully, and in that the winding is carried out in such a manner that the expansion tape on one conductor makes contact both with the expansion tape on adjacent conductors and with the expansion tape on adjacent filler strings within a given length of cable.
This allows to use cheaper expansion tape as it is not necessary anymore to have it semi-conducting, while keeping the water sealing function.
According to a preferred embodiment, the direction of winding of the expansion tape on the conductors is opposite to the direction of winding on the filler strings. This is one of the possible solutions for ensuring contact between expansion tapes on the conductors and on the filler strings.
According to another embodiment of the present invention, the pitches with respect to the winding of the expansion tape are different on the three conductors. This is another solution for ensuring contact between expansion tapes on the conductors.
In such a case, the pitches with respect to the winding of the expansion tape are different on the filler strings.
According to still another embodiment, the pitches of the winding of the expansion tape have the same basic value but vary randomly within predetermined limits.
The expansion tape may be made of non semi-conducting material.
The expansion tape may be made of two woven layers with a layer of expansion powder between them.
At last, the filler strings that have been wound with expansion tape comprise filler tubes that have obtained the intended shape on cabling. This allows to have a better round shape of the cable once finished.
The invention will now be described in more detail with reference to the attached drawings, which also show, as described above, the principles of known constructions of three-core cables with a longitudinal water seal as comparison with the principles of the new design of a three-core cable of the invention.